Our goal is to develop a new technology for non-invasive optical monitoring of activity of individual retinal neurons and their light-driven inputs at cellular resolution, in the living human retina. If successful, this technology will provide an entirely new and objective approach to understand and monitor treatment of retinal disease, thereby transforming scientific studies of the eye and vision. This project directly addresses the priorities outlined in the RFA-EY-14-001, the first RFA within the NEI Audacious Goal Initiative. The proposed work relies on combining and validating two new approaches. First, interferometry (including phase-resolved OCT; Park Lab at UC Riverside) can, in principle, be used to measure nanometer-scale distortions in the membranes of cells that occur during membrane depolarization and ion influx. With depth resolution, these measurements will enable us to measure neural activity non-invasively, throughout the layers of the retina, at cellular resolution. Second, adaptive optics scanning laser ophthalmoscopy (Roorda Lab at UC Berkeley) and image-based eye tracking can be used to position stimulating and measurement beams on the retina with cellular precision in the living eye, by overcoming optical aberrations and eye jitter. This technology will allow us to activate individual photoreceptors and groups of photoreceptors with visible light while imaging the resulting electrical activity of individual downstream cells, in vivo. To advance and combine these approaches requires a stepwise aggregation of technology. In a unique collaboration, we will build on simpler wide-field interferometric measurements of electrical activity in isolated retina (Palanker Lab at Stanford University), combined with large-scale multi-electrode physiological measurements in primate retina (Chichilnisky Lab at Stanford University) to validate and tune the optical measurements. Ultimately, the innovation at each step forms a powerful tool, independently or with a combination of other approaches, and finds applicability to optical imaging, retinal physiology, psychophysics and clinical ophthalmology. The specific aims are: Aim 1. Wide-field interferometry for measuring patterns of neural activity in primate retina Depolarization during neural signaling produces nanometer-scale deformations in cells that are detectable with interferometry. The simplest approach is wide-field interferometric microscopy with transmission geometry in isolated retina. We will measure depth-resolved optical phase changes produced by neural activity in primate retina, and use them for physiological characterizations of many retinal ganglion cells (RGCs) and other retinal neurons simultaneously. Aim 2. Phase-resolved OCT for reflectance measurements of patterns of retinal activity. The next step toward human application is phase-resolved OCT; essentially, low-coherence interferometry and a well-established tool for in vivo imaging. We will record optical path length changes associated with neural activity in reflection geometry using point-scanning, near-IR (1060 nm), phase-resolved OCT on isolated primate retina. Aim 3. Adaptive optics, eye tracking and phase-resolved OCT for measuring human retinal function. Deployment in humans requires compensating for optical aberrations in the eye as well as eye movements. We will develop a system that uses AOSLO to image the retina for eye tracking, targeted delivery of stimulation light, and positioning of the OCT probe. We will test this system in humans and demonstrate its potential application in clinical settings.